Electronic photodissociation spectra of the Ar n C 4 H þ 2 weakly bound cationic complexes

Transcription

1 Journal of Molecular Spectroscopy 222 (2003) Electronic photodissociation spectra of the Ar n C 4 H þ 2 weakly bound cationic complexes (n ¼ 1 4) T.W. Schmidt, T. Pino, J. van Wijngaarden, K. Tikhomirov, F. G uthe, and J.P. Maier * Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Received 2 September 2002; in revised form 30 December 2002 Abstract Electronic spectra of a series of weakly bound clusters consisting of argon (Ar n, n ¼ 1 4) bound to the butadiyne cation, C 4 H þ 2, have been recorded in the visible range from 440 to 520 nm by photodissociation. The C 4 H þ 2 fragment signal was recorded as a function of the laser wavelength during excitation of the A X electronic transition. The observed transitions were assigned to the band origin of the cationic complexes and to vibronic bands involving excitation of the m 3 and m 7 vibrational modes of the C 4 H þ 2 moiety, as well as combination bands of these modes. Comparison of the photodissociation spectra of the various clusters reveals a small blue shift, 25 cm 1 of the band maxima relative to the corresponding transitions reported from gas phase spectra of the bare C 4 H þ 2 cation. The magnitude of the blue shift of each band increases with successive Ar solvation up to n ¼ 3. Furthermore, each band becomes increasingly broadened towards the red with the addition of Ar atoms due to an increasing number of unresolved transitions involving excited intermolecular modes. Ó 2003 Elsevier Science (USA). All rights reserved. PACS: 36.40; R Keywords: Clusters; Optical spectroscopy 1. Introduction Polyynic cations, such as HðCCÞ n H þ, have been of interest to spectroscopists and theoreticians due to their abundance in flames and plasmas. A number of these species have also been investigated as possible carriers of the diffuse interstellar bands (DIBs) since both neutral acetylene and butadiyne have been previously observed in planetary atmospheres [1,2] and circumstellar envelopes [3]. In particular, the butadiyne cation, C 4 H þ 2,has received much attention since its emission spectrum was first observed in 1951 [4] and later assigned by Callomon [5]. Since those preliminary works, several other gas phase spectroscopic studies have confirmed the assignment of transitions in the A 2 P u X 2 P g band system of C 4 H þ 2 and have extended the understanding of the spectroscopy of this species [6 12]. These gas phase * Corresponding author. address: j.p.maier@unibas.ch (J.P. Maier). results are complemented by electronic spectra recorded in solid rare gas matrices [13 15] as well as theoretical predictions of the spectra [14,16,17]. In general, the spectroscopy of molecular cations is less characterized than for neutral molecules and their anions. This is a result of the inherent difficulties associated with measuring spectra of polyatomic cations. For instance, it is necessary to have a source that produces sufficient quantities of the desired cationic species, preferably vibrationally and rotationally cold. Early studies of polyatomic cations involved measuring emission spectra following their production via electrical discharge or electron impact ionization of an effusive beam. For larger molecules, however, there are an increasing number of competing non-radiative pathways for electronic relaxation and thus measuring fluorescence becomes unfeasible. Methods that are applicable to a broad range of polyatomic cations are therefore of great interest. A promising technique that overcomes these aforementioned difficulties involves the production of weakly bound complexes of the desired species and rare gas /$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi: /s (03)

2 T.W. Schmidt et al. / Journal of Molecular Spectroscopy 222 (2003) atoms. After complexation, laser excitation may be used to photodissociate the weakly bound system as demonstrated for a variety of aromatic cations [18 21]. The appearance of a fragment as a function of the excitation wavelength can be detected via mass spectrometric methods and provides evidence of photon absorption by the parent cluster. The transitions observed from the photodissociation of weakly bound clusters are shifted relative to the spectrum of the bare cation. These spectral shifts are typically very small, on the order of tens of wavenumbers, and have been shown to be additive with the successive addition of Ar atoms. Should the binding sites for successive argon atoms added be equivalent, then the shifts will be the same. This suggests that electronic spectra obtained via photodissociation of weakly bound complexes can provide a means to extrapolate the gas phase resonant frequencies of the bare cations. The general aim of this work is to develop an experimental scheme in our laboratory for measuring the visible spectra of a broad range of carbon containing cations via a mass selective photodissociation technique. The potential species of interest are hydrocarbon rings and chains which have been proposed as carriers of the DIBs [22]. Consequently, the technique developed must yield spectra from which direct comparison with astrophysical observations may be made. More specifically, the following paper describes the measurement of the electronic spectra of our prototype species for these photodissociation studies, the Ar n C 4 H þ 2 (n ¼ 1 4) cationic clusters produced via electron impact ionization of the neutral species. This source produces cold, isolated cations and thus approaches the conditions in interstellar environments. The spectra are produced using mass spectrometric detection of C 4 H þ 2 fragments upon photodissociation of the parent weakly bound clusters. The Ar n C 4 H þ 2 complexes, in particular, are good test molecules for this photodissociation technique since the electronic spectrum of C 4 H þ 2 is well known in the gas phase and in solid rare gas matrices. Furthermore, recent theoretical calculations for the Ar C 4 H þ 2 cation [23] provide a means for comparison with the current experimental results. 2. Experimental The experimental setup couples a reflectron tandem time-of-flight mass spectrometer with a tunable laser. This technique involves detection of the interaction between a mass selected weakly bound cluster cation and a laser beam, and is similar to that implemented by previous investigators [24,25]. The primary ions are mass selected in a time-of-flight mass spectrometer (TOF1). Upon interaction with the desired species, the laser beam causes photodissociation of the primary ions, resulting in secondary ions of reduced kinetic energy. The secondary ions are then reflected by a reflectron time-offlight mass spectrometer and detected by a multichannel plate. Setting the reflectron voltage appropriately ensures that only secondary ions are detected. Scanning the laser frequency reveals the photodissociation spectrum of the desired species. A schematic of the experiment is shown in Fig. 1. The Ar n C 4 H þ 2 species were produced using a supersonic molecular beam electron impact source with a gas mixture of 0.1% butadiyne in argon. This source has been shown previously to yield a variety of weakly bound cluster ions [26]. The butadiyne was synthesized according to the method described by Brandsma [27]. The mixture was fed into the source with backing pressure of 10 bar. An electromagnetic valve opened the nozzle at a frequency of 20 Hz synchronized with the frequency of the laser. The gas mixture passed into the chamber where it interacted with an electron beam formed by two W-Th filaments placed approximately 3 mm downstream from the orifice of the nozzle. A continuous current of 6 A was passed through the filaments and a repeller voltage of 160 V was used to accelerate the electrons. The nozzle and electron impact source were cooled to )3 C by a cryostat operated external to the vacuum chamber. Species generated in the impact source expanded into the chamber, where collisional cooling resulted in the formation of clusters. The pulsed molecular beam was collimated through a 2 mm grounded skimmer before passing into the extraction region of the Wiley McLaren time-of-flight mass spectrometer (TOF1). Positively charged species were accelerated by a potential of 2.7 kv into the flight tube, where the horizontal and vertical position of the beam was adjusted using deflection plates. The beam was focused by an ellipsoidal einzel lens in order to maximize its overlap with the laser. The focussed ion bunch was intersected perpendicularly by the laser beam, which was overlapped in time and space with the desired species. Photodissociation resulted in production of fragment ions of the same velocity as the primary ions, but with a reduced kinetic energy. At the end of the flight tube, the ion beam entered the reflectron part of the mass spectrometer (RE- TOF2). For the detection of a normal time of flight mass spectrum, the potential between the grids of the reflectron was set to 2.7 kv, so that all the species were directed to the MCP detector. To detect the products of photodissociation, the reflecting potential was set to 0:80 1:56 kv depending on the probed species, so that only C 4 H þ 2 fragments were reflected towards the MCP. The undissociated species possessed enough kinetic energy to traverse the grids and were not detected. The laser system used was a commercial optical parametric oscillator (bandwidth 0.05 cm 1 ) pumped by the third harmonic of a Nd:YAG laser. The energy

3 88 T.W. Schmidt et al. / Journal of Molecular Spectroscopy 222 (2003) Fig. 1. Schematic of the experimental setup. Ar n C 4 H þ 2 ions are produced in the supersonic jet electron impact source. The beam is skimmed and extracted into a time-of-flight mass spectrometer (TOF1) where an ion bunch of specified mass interacts with a tunable laser. The C 4 H þ 2 fragments are reflected (RE-TOF2) by a lower voltage than that used to extract the main beam, and detected by an MCP. The dashed lines show the trajectory of the ion beam. density of the laser was maintained at less than 5mJcm 2 to suppress multiphoton processes and reduce power broadening of the spectra. The overview spectra of Ar n C 4 H þ 2 were recorded with a wavelength step size of 0.1 nm and the individual bands with a laser step size of 0.03 nm. The spectra were averaged over 40 pulses of the laser operating at 20 Hz. The MCP signals were digitized by an oscilloscope and downloaded to a personal computer where the signal was processed and stored as a spectrum. Where necessary, multiple scans were averaged to yield the spectra. energy is in any case limited to the photon energy less the cluster dissociation energy, which is three orders of magnitude less than the kinetic energy of the parent cluster ion (2.7 kev). For the Ar n C 4 H þ 2 species, should dissociation occur according to Eq. (1), the threshold 3. Results and discussion 3.1. Overview and band assignment Fig. 2 displays the photodissociation spectra of the Ar n C 4 H þ 2 family of cations. The spectra are plotted such that the corresponding peaks in each spectrum are of similar height. Manipulation of the reflectron voltage confirmed that the signal measured arose from the process Ar n C 4 H þ 2 þ hx! C 4H þ 2 þ nar; ð1þ whereby the signal is carried by the bare C 4 H þ 2 cation impinging upon the MCP. In the limit of zero recoil energy, fragments possess a fraction of the kinetic energy of the parent in proportion to their mass. The recoil Fig. 2. An overview of the dissociation spectra of Ar n C 4 H þ 2, n ¼ 1 4. The band assignments are discussed in the text.

4 T.W. Schmidt et al. / Journal of Molecular Spectroscopy 222 (2003) voltage for the detection of fragments may be expressed as m C4 H KE thresh ¼ V þ 2 TOF1 m C4 H þ þ m Arn ¼ 2:7keV þ 40n : ð2þ 2 In order of ascending Ar n C 4 H þ 2 cluster mass, (n ¼ 1 4), the threshold reflectron voltages for reflection of the C 4 H þ 2 fragment are thus 1.5, 1.0, 0.80, and 0.64 kev. In each case, the applied reflectron voltage was kept at or just above the threshold for C 4 H þ 2 detection. The fragments were detected at the same time-of-flight as that of the parent ions whilst the reflectron voltage was set to 2.7 kv. Surveying the entire spectral range of each species reveals the origin band and a number of roughly evenly spaced transitions. The bands correlate to absorptions by the chromophore, C 4 H þ 2 [5]. The notations used for the intramolecular modes are given as for the bare cation [5]. The assignments are the same for the four clusters. Band I is assigned as the A X band origin. Bands II and III are assigned as the and the 72 0 bands. These represent a quantum of C C stretch (806 7cm 1, n ¼ 1) and two quanta of bending motion (862 7cm 1, n ¼ 1), respectively. These values compare favourably with those of the bare cation. Since C 4 H þ 2 is rigorously linear, only even quanta of bending motion in the excited state can be accessed from the ground vibrational level of the X state. Perturbation of the energy levels by the argon ligands is not observed to be sufficient to allow population of a single quantum of the m 7 bending mode. Bands IV and V are assigned as polyads of even quanta of the bend, m 7, and a corresponding number of quanta of stretch, m 3. The individual components of the polyads could not be resolved. By saturating the photodissociation spectrum of Ar C 4 H þ 2, other transitions are seen. These are assigned to other bending modes, such as m 6, m 8, and m 9, populated with even number of quanta. The transitions 9 2 0,62 0, and are observed with frequencies relative to the band origin of 456 7, , and cm 1. Additionally, a single peak corresponding to excitation of an acetylenic stretching vibration was seen cm 1 higher than the origin band. This has been previously assigned to the transition [10], the symmetric acetylenic stretch, but may also correlate to 5 1 0, the corresponding asymmetric stretch in the bare C 4 H þ 2 cation, should the argon ligand induce a large distortion from D 1h symmetry. In any case the symmetric stretch should be observed and thus the band in question is assigned here as The bands observed in the spectra of the series Ar n C 4 H þ 2 (n ¼ 1 4) are listed in Table 1. For clusters containing more argon atoms, the signal levels precluded the ability to search for the transitions correlating to those forbidden in C 4 H þ 2. Table 1 Measured positions in cm 1 (5cm 1 ) and assignments Label Transition n ¼ 1 n ¼ 2 n ¼ 3 n ¼ 4 I II III IV (polyad) V (polyad) The 3 1 0,72 0 polyad was unresolved for n ¼ 3. Other polyads were unresolved and are listed in the table as m 3 transitions. For the n ¼ 4 cluster, the values should be taken as indicative only Line shifts and band profiles Each band is broadened upon the successive addition of argon atoms. Fig. 3 displays the band profiles of the origin band of the Ar n C 4 H þ 2 clusters, n ¼ 1 3. In each, the band is asymmetrically broadened to the red. This is explained in terms of hot bands corresponding to excited intermolecular modes. According to Botschwina [23], ab initio calculations at the RCCSD(T) level of theory employing the aug-cc-pvqz basis set reveal a well-depth for the Ar C 4 H þ 2 complex in the linear geometry of 560 cm 1. This may be taken as an upper limit for the extent to the red for which hot bands may Fig. 3. The spectral features of the Ar n C 4 H þ 2 (n ¼ 1 3) spectra. Upon addition of argon atoms the peaks are shifted to higher frequency and broadened substantially. The bands remain asymmetric upon addition of argon atoms.

5 90 T.W. Schmidt et al. / Journal of Molecular Spectroscopy 222 (2003) be seen. In Fig. 3, for Ar C 4 H þ 2, a broadening of approximately 200 cm 1 is seen to the red, compared with 60 cm 1 to the blue (taken as position where signal is 10% of peak maximum). Upon saturation, the hot band structure is more expressed and continues as far as 510 cm 1 to the red. The broadening to the blue remains approximately the same, irrespective of laser intensity. The transitions on the blue side of the peak correspond to excitations of the intermolecular modes in the excited state. It is worth noting that the rotational profile of each vibronic transition in Ar C 4 H þ 2 extends only 5cm 1 (implementing the ab initio equilibrium geometry [23]), even at a rotational temperature as high as 50 K (far higher temperature than expected for a supersonic jet expansion). The band structures observed are thus due to intermolecular modes of the complexes. Upon addition of more argon atoms to the complex, the broadening extends further to the blue and to the red, yet the asymmetric nature of the band remains. While in the larger complexes the hot band structure is more pronounced, it never extends more than 560 cm 1 to the red (the calculated well-depth [23]). Fig. 2 reveals a broad structure for the origin band for the Ar 4 C 4 H þ 2 cluster, the centre of which is to the red of that of the bare cation. In Fig. 4, the feature corresponding to the and 72 0 transitions of the chromophore are shown for the Fig. 4. The =72 0 spectral features of the Ar n C 4 H þ 2 (n ¼ 1 3) spectra. Upon addition of argon atoms the peaks are shifted to higher frequency and broadened substantially. The bands remain asymmetric upon addition of argon atoms. The double peaked structure visible for n ¼ 1; 2 disappears for n ¼ 3. This is possibly due to a shift in the bending frequency (see text). Ar n C 4 H þ 2 series (n ¼ 1 3). This structure must be considered as the superposition of two bands, one for each of the aforementioned transitions. While these two bands may be partially resolved for n ¼ 1; 2, they are not for n ¼ 3. With the double peaked structure in mind, it is seen that the behaviour of each peak with addition of successive argon atoms is similar to that of the origin band. In any particular band, the signal is proportional to the population in the lower state of the transition of interest. The density of intermolecular modes on the X state surface is expected to grow rapidly with increasing energy and cluster size. At thermal equilibrium, therefore, the population maximum will not be at zero vibrational energy, but at a compromise between density of states and Boltzmann weighting. A combination of these effects may explain the growing structure on the lower frequency side of the origin bands, and why the band maxima for Ar 4 C 4 H þ 2 are observed to be at lower frequencies than those observed for the bare C 4 H þ 2 cation (in contrast to the other Ar n C 4 H þ 2 clusters measured). Noticeable in Fig. 2 is a general shift to higher frequency, of the maxima of the bands corresponding to the origin band of C 4 H þ 2,of25 cm 1 per argon atom. If the maxima of these bands correspond to the origin band for each weakly bound complex, then it is concluded that for all of the complexes, Ar n C 4 H þ 2, the excited state is more weakly bound than the ground state. Due to the absence of peaks corresponding to single quanta of bend (m i, i ¼ 6 9), there is no evidence to suggest that the Ar C 4 H þ 2 cluster is anything but linear, as predicted by ab initio calculation [23], and as seen for species such as Ar HCO þ [28]. No splittings are observed in the spectrum that can be ascribed to the effect of spin orbit or Renner Teller coupling due to the 2 P nature of the electronic states involved in this electronic transition. As such, there is no indication, so far as lifting the degeneracy of the P state is concerned, to suggest a ÔT-shapedÕ molecular structure for Ar C 4 H þ 2. In Ar 2 C 4 H þ 2, the band structures remain similar to those of Ar C 4 H þ 2. This implies that the binding site is equivalent. It is thus likely that this complex also has a linear structure. Addition of the third argon atom must then alter the pattern such that argon atoms are positioned a way that they may interfere with the bending modes of the chromophore. There is evidence for such behaviour in the band seen for n ¼ 3 in Fig. 4, whereby the double-peaked structure of the diad is lost. This may be an indication of a lowering of the 2m 7 bending frequency such that it overlaps with the C C stretch. The reappearance of the double peaked structure for Ar 4 C 4 H þ 2 suggests that the bending frequency is lowered to an extent that the 2m 7 band appears to the red of the m 3 band, contrary to the assignments given in the table. Despite the lack of definitive spectral information,

6 it is clear that intermolecular vibrational modes are responsible for the observed band profiles Dynamics T.W. Schmidt et al. / Journal of Molecular Spectroscopy 222 (2003) In addition to the electronic oscillator strengths, Frank Condon factors and lower state populations for each vibronic transition, the signal is proportional to the quantum yield for complete evaporation of the argon atoms. Upon excitation of the C 4 H þ 2 chromophore to the ground vibrational state of the A state, there is no vibrational energy with which to expel argon atoms. This molecule has been seen to fluoresce [10] to many vibrationally excited levels of the X state with a lifetime of 72 ns [29]. Comparison of this value with the observed oscillator strength reveals a fluorescence quantum yield of / F ¼ 0:72. There are thus two pathways to photodissociation; fluorescence to excited vibrational levels of the ground state, or direct non-radiative crossing to highly vibrationally excited X state levels. Since a significant proportion of the fluorescence is directed to the ground vibrational level of the X state, the quantum yield for C 4 H þ 2 production is less than unity. In contrast, excitation to vibrationally excited levels of the A state is expected to result in evaporation of the argon atoms on a time scale much shorter than the observed radiative lifetime of 60 ns [29]. Whereas the origin band is seen to be stronger than the band in laser induced fluorescence studies of the C 4 H þ 2 ion, for the Ar n C 4 H þ 2 series (this work) these bands are of approximately equal size. The quantum yield of C 4 H þ 2 from Ar n C 4 H þ 2 in the origin band may be estimated to be approximately Extrapolation of observed bands The principal motivation of this work was to develop methods for locating optical resonances of carbon chain cations by dissociation of their weakly bound clusters. This approach has been applied successfully by previous investigators to PAH cations such as the napthalene cation [18,19]. In Fig. 5, the band maximum is plotted as a function of n, the number of argon atoms attached to the C 4 H þ 2 chromophore. As discussed above, the shifts are only expected to be of a similar magnitude if there is an equivalence of binding sites. In the present case, this can only be assumed for n ¼ 1 2. The points corresponding to the band maxima for Ar 3 C 4 H þ 2 and Ar 4 C 4 H þ 2 were not included since their positions did not reflect the effect of an argon atom binding to equivalent sites as for n ¼ 1 2. By extrapolation to n ¼ 0 of the points for n ¼ 1; 2, it is found that the value of cm 1 encompasses the measured band origin of the bare cation, cm 1 [10] which is consistent with the above interpretation. This extrapolation of band position rests heavily on the interpretation of the Fig. 5. The band maxima of the regions of the photodissociation spectra of the ionic clusters Ar n C 4 H þ 2. The line drawn is through the data for n ¼ 1 2. The extrapolated band position at n ¼ 0(C 4 H þ 2 )is shown as cm 1 and is compared to the measured origin of the bare cluster, cm 1. band maximum as being the origin position. In lighter molecules than those studied by others [18,19,28], the argon atoms have a profound effect upon the band profile [30]. Without the aid of a rigorous theoretical interpretation of the band profiles, and cooler conditions than those achievable in the present experiment, one must regard the extrapolated position of the band maximum in Fig. 5 as merely an estimate of the position of the origin band of the bare cation. 4. Conclusions Analysis of the photodissociation spectra of Ar n C 4 H þ 2 (n ¼ 1 4) reveals that the A states of these species are more weakly bound with respect to removal of argon than the X states. As a consequence, the band positions are shifted to the blue by 26 cm 1 per argon atom (n ¼ 1 3), and the band profiles are asymmetrically broadened to the red due to hot band structure. Experimental and theoretical evidence points to a linear structure for the Ar C 4 H þ 2 and Ar 2 C 4 H þ 2 clusters. A plot of the number of argon atoms against the position of the band predicts a range of the position of the origin band in which the measured value of the bare cation falls. The usefulness of this technique for estimating the positions of the band origins of molecules of astrophysical interest is limited by the interpretation of the band profiles of the weakly bound complexes. The present technique is expected to be useful for molecules possessing transitions which are broadened to such an

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